The field of the invention is nuclear magnetic resonance imaging ("MRI") methods and systems. More particularly, the invention relates to the production of magnetic field gradients in MRI systems.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. An NMR signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated and, this signal may be received and processed to form an image.
When utilizing NMR signals to produce images a technique is employed to obtain NMR signals from specific locations in the subject. Typically, the region which is to be imaged (region of interest) is scanned by a sequence of NMR measurement cycles which vary according to the particular localization method being used. The region of interest may be a small portion of a patient's anatomy, such as the head or heart, or a much larger portion, such as the entire thorax or spine. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. To perform such a scan, it is, of course, necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing magnetic fields (G.sub.x, G.sub.y, and G.sub.z) which have the same direction as the polarizing field B.sub.0, but which have a gradient along the respective X, Y and Z axes. The magnetic field gradients are produced by a trio of coil assemblies placed around the object being imaged. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified.
In order to accommodate the imaging of large portions of a patient, each gradient field coil must produce a magnet field that varies linearly along one axis of a very large volume. On the other hand, to image a small portion of a patient, each gradient field coil may be smaller in physical size and have entirely different electrical characteristics than the larger whole-body gradient coils.
There are many conflicting design considerations when providing an optimal gradient subsystem for an MRI system. Factors such as peak gradient amplitude, peak gradient slew rate, gradient spatial linearity over the imaging volume, heat generation and patient safety with respect to peripheral nerve stimulation (i.e. dB/dt limits) all must be considered. Compromises must be made. For example, a very linear field gradient coil tends to be high in inductance and requires a high voltage power supply to provide a high slew rate. A coil which produces a linear gradient field over a large field of view for spinal imaging can produce excessive dB/dt when driven at a high slew rate. The efficiency (and hence heat generation) is better when a smaller gradient coil having a reduced diameter or shorter length is employed. Smaller diameter gradient coils are used, for example, when imaging the head with fast EPI pulse sequences.
A number of solutions have been proposed to address this gradient subsystem design dilemma. As described in U.S. Pat. No. 5,311,135, for example, the gradient coil windings may be tapped and the gradient amplifiers can be switched to different taps on the coils depending on the particular scan being conducted. Such switching changes the size and location of the optimal gradient fields as well as the electrical characteristics of the coils. In another solution described in U.S. Pat. No. 5,736,858, two separate sets of gradient coils are provided and the three gradient amplifiers may be switched to either or both sets depending on the particular scan being performed. One set of gradient coils is a relatively large, whole-body coil and a supplementary gradient coil set is relatively small. The gradient amplifiers have fast semiconductor switches capable of rapidly switching between three gradient coil configurations within a pulse sequence. These electronic switches add a substantial complication to the design.